Snow Leopard patches were unexpected. In the past, Apple has stopped serving security updates for one edition around the time it has two newer in play. If the current OS X is dubbed "n," then "n-2" support ends at the debut of "n."

In that scenario, Mountain Lion, which Apple launched in July, serves as "n," and 2009's Snow Leopard is "n-2."

Apple has never put its support practices on paper or on its website, so customers and security experts must wait for the next round of updates to confirm their suspicions that an edition has been retired, or find out that support has been extended.

Yesterday, Apple updated both Mountain Lion and Lion with new features, improvements to existing ones, bug fixes and more than 30 patches for critical vulnerabilities.

Before Wednesday, Apple's last Snow Leopard security update shipped on May 9.

Because of yesterday's update, only a small minority -- about 13% -- of all Macs are running a retired edition of OS X. If Apple had done the expected and not patched Snow Leopard, that number would have ballooned to 47%, according to the latest statistics on operating system usage from metrics company Net Applications.

"Apple realized they need to support Snow Leopard because there are still so many active installations," said Wolfgang Kandek, CTO of Qualys, in reply to questions posed via instant message. "It might be a Windows XP-like effect. Snow Leopard works really well, and many customers have not migrated off it."

As Kandek noted, Microsoft has had difficulty convincing users of the 11-year-old Windows XP to upgrade to a new edition; by Net Applications' numbers, XP powered 46% of all Windows PCs last month.

It's not clear whether Wednesday's Snow Leopard update was a one-off -- Apple never comments on security matters -- or an indication that the company plans to support OS X 10.6 for an extended period.

One clue to the latter was the eight-week stretch between the launch of Mountain Lion and yesterday: The previous Apple record for shipping an update after two newer versions of OS X have been in play was in 2007, when Apple delivered the final patches for OS X Panther 19 days after the launch of Leopard.